IGBTs are minority carrier devices that have high input impedance and large current gain. The high input impedance is provided by a metal-oxide-semiconductor input stage and the large current gain by a bipolar output stage. IGBTs are voltage-controlled bipolar devices that exploit the advantages of both metal-oxide-semiconductor transistor (MOSFET) and bipolar junction transistor (BJT) technologies.
IGBTs are typically used in power electronics applications where switching of large voltages and currents is needed. For example, IGBTs can be used in bridge circuits of power converters or in pulse width modulators for three-phase drives. In these applications, IGBTs must be turned off quickly in order to minimize switching losses and the devices' dead time.
In the aforementioned applications and in others where IGBTs are typically used, a particular circuit branch can include two IGBTs placed across a common source, with one IGBT being on at a time. When either one of the IGBTs fails, and the other one is turned on, a short circuit is created, causing a large current to be drawn into the circuit. This large and sudden change in current depends on the inductance of the circuit branch, and it can induce a voltage large enough to prevent the IGBT that is being turned on from reaching saturation, i.e. from reaching the biasing condition where it is fully turned on. This condition is called “desaturation,” and when it occurs, an IGBT must be turned off.
However, during de-saturation, turn-off is challenging, as a large collector-to-emitter voltage is developed across the IGBT, in addition to the large current being drawn. These conditions cause large voltage overshoots, which can damage the device and/or its loads.
Desaturation is typically handled by introducing a Zener diode across the IGBT's collector-to-emitter branch to block the short circuit current. However, this method reduces the DC link voltage at which the IGBT can be operated, thus preventing it from being used for relatively high power applications. As such, there is a need for gate drive technologies that provide adequate means for handling and preventing damage that can potentially occur from desaturation without limiting the devices' power delivery capacity.